who

Print all usernames currently logged on, showing for each user
the login name, tty name, the date and time of login, and hostname if not local.

Syntax
who [-mTuH] [file]
who am i
Options -m Only print information about the current terminal.
This is the POSIX way of saying who am i.
-T Print a character after the user name indicating the state of the
terminal line: `+' if the terminal is writable; `-' if it is not;
and `?' if a bad line is encountered.
-u Print the idle time for each user.
-H Write column headings above the regular output.
am I Returns the invoker's real user name.
file By default, who gathers information from the file /var/run/utmp. An
alternative file may be specified which is usually /var/log/wtmp
(or /var/log/wtmp.[0-6] depending on site policy as wtmp can grow
quite large and daily versions may or may not be kept around after
compression by ac(8)). The wtmp file contains a record of every
login, logout, crash, shutdown and date change since wtmp was last
truncated or created.

NotesIf /var/log/wtmp is being used as the file, the user name may be empty or
one of the special characters '|', '}' and '~'. Logouts produce an out- put
line without any user name. For more information on the special characters,
see utmp(5).

“The more the merrier” ~ John Heywood

Related:

who man page - Apple.com
last - indicate last logins of users and ttys
mesg - display (or do not display) messages from other users
users - Print login names of users currently logged in